Tribological Performance for Steel–Steel Contact Interfaces Using Hybrid MWCNTs/Al 2 O 3 Nanoparticles as Oil-Based Additives in Engines

: Numerous problems occur during engine operation, such as start-up, lack of lubrication, and overheating, resulting in engine components’ wear, power loss, and fuel consumption. Nanoma-terials dispersed in engine oil can play an important role in improving the tribological properties of oil lubricants. This study investigated the inﬂuence of multi-walled carbon nanotubes (MWCNTs) and aluminum oxide nanoparticles (Al 2 O 3 NPs) as nano-additives for lubricants. Different engine oil samples were loaded with 0.5–2.0 wt% Al 2 O 3 NPs and 0.5–1.0 wt% MWCNTs and compared with unmodiﬁed oil. The tribological performance of the nano lubricants was investigated using the four-ball test method. In addition, the wear scar in the engine was evaluated using 3D micrographs and scanning electron microscopy (SEM). The results of the sliding surfaces with hybrid MWCNTs/Al 2 O 3 NPs showed better friction performance and wear resistance. The coefﬁcient of friction (COF) and wear scar width were improved by 47.9% and 51.5%, respectively, compared with unmodiﬁed oil.


Introduction
Reducing fuel consumption and energy losses is an interesting task for many users of internal combustion engines. It has been proven that power losses account for up to 20% of the total energy generated by automotive engines [1][2][3][4]. Friction among the internal parts of the engine causes about 35-45% of power losses [5,6]. Lubricants are substances typically used to reduce friction between sliding parts. Depending on their properties, lubricants also contribute to the efficient and unambiguous operation of various mechanisms [7,8]. Therefore, nano-additives have attracted much attention due to their unique mechanical, physical, and chemical properties [9][10][11]. The dispersion of nanomaterials in lubricants contributes significantly to the improvement of tribological properties, and the friction and wear performance of contact surfaces [12][13][14][15]. Many studies have shown that the use of hybrid nano-additives (Al 2 O 3 , ZnO, TiO 2 , MoS 2 , SiO 2 , CuO, Ag, carbon nanotubes, and graphene oxide) has a noticeable effect on the thermal and tribological properties of lubricants [16][17][18][19][20][21][22][23][24][25][26][27]. The mechanism of nanoparticles has unique properties to reduce friction and wear. These properties are the rolling effect, the formation of tribofilm, the repair property, and the abrasive effect [28,29]. Oil nano-additives aim to improve the frictional behavior of sliding surfaces, which contributes to the reduction in power loss and fuel consumption [30][31][32][33][34]. Nanoparticles (NPs) play an active role in heat transfer, leading to an improvement of the tribological mechanism under heat stress boundary

Materials and Experimental Setup
The present work aimed to study the influence of the dispersion of hybrid nanofillers in 10W30 engine oil. The technical properties of the lubricating oil are presented in Table 1. Al 2 O 3 NPs and MWCNTs were supplied by US Research Nanoparticles, Inc. Al 2 O 3 NPs had a diameter of 50 nm, a specific surface area of 15 m 2 /g, and a density of 3.97 gm/cm 3 , while MWCNTs had different dimensions, on average 30-50 nm outer diameter, 5-12 nm inner diameter, a length of 10-20 um, an SSA of 60 m 2 /g, and a density of 2.1 gm/cm 3 . Figure 1 show the SEM images and X-ray diffraction (XRD) patterns of Al 2 O 3 NPs and MWCNTs, respectively. The XRD pattern of Al 2 O 3 NPs shows that the diffraction peaks are centered at 2θ of 32.7 • , 39.6 • , and 63.7 • , which could indicate crystal planes. The XRD analysis of MWCNTs shows a diffraction peak at 2θ of 26.5 • , indicating the presence of carbon. The XRD pattern shows peaks with low intensity. Oleic acid was used to improve and develop the dispersion of nanoparticles in lubricating oil. Lubricant samples containing 0.5, 1.0, 1.5, and 2.0 wt% Al 2 O 3 -NPs and 0.5 and 1.0 wt% MWCNTs were prepared. The compositions of the samples are listed in Table 2. The nanofillers were incorporated into the lubricating oil by stirring for 2 h at a constant speed of 300 rpm at a temperature of 30 • C and a relative humidity of 55%. Oleic acid, as a surfactant, helped to stabilize the suspension and dispersion of the nanofillers in the oil and to reduce the occurrence of the agglomeration of the nanoparticles [55]. The samples were stored for two months after preparation to monitor the stability of the nanofillers. The viscosity of the lubricant samples was determined using a B-One plus viscometer with spindles. The experiments were based on ASTM standard D446. The kinematic viscosity of the samples was determined at the temperatures of 40 • C and 100 • C. The coefficient of friction (COF) and the wear rate were determined using a block ring tribometer. The tribological performance of the nano lubricant samples was evaluated according to the procedures of ASTM G77-17. The tribometer was set with speeds of 500, 1000, 1500, 2000, and 2500 rpm at a load of 200 N. The samples were tested at temperatures of 50, 75, and 100 • C to simulate operating conditions. For each oil sample, the steel block was replaced to avoid overlapping the experimental results. Each experiment was repeated five times, and the standard deviation was reported. The wear mechanism was performed under different working conditions, speeds, and temperatures. The surfaces of the blocks were cleaned with acetone after each test to ensure that the deposits were removed. The wear rate of the tested steel block was estimated based on the following relationship: where ∆V is the accumulated lost volume of the tested steel block (mm 3 ). It can be estimated with the ratio of ∆m/ρ, where ∆m is the weight loss (gm), determined by weighing the steel block before and after testing, and ρ is the steel density (gm/mm 3 ). F n is the normal force, and L is the total sliding distance recorded during the test. In addition, the wear tracks and marks of each block were evaluated using an electronic microscope (OLYMPUS BX53M; Tokyo, Japan). The morphology of the worn surfaces was examined with a scanning electron microscope (SEM; JCM-6000Plus; JEOL, Tokyo, Japan). The viscosity of the lubricant samples was determined using a B-One plus viscometer with spindles. The experiments were based on ASTM standard D446. The kinematic viscosity of the samples was determined at the temperatures of 40 °C and 100 °C. The coefficient of friction (COF) and the wear rate were determined using a block ring tribometer. The tribological performance of the nano lubricant samples was evaluated according to the procedures of ASTM G77-17. The tribometer was set with speeds of 500, 1000, 1500, 2000, and 2500 rpm at a load of 200 N. The samples were tested at tempera-

Influence of Al 2 O 3 /MWCNTs on Kinematic Viscosity
The effect of the dispersion of the hybrid Al 2 O 3 /MWCNTs in lubricating oil on the kinematic viscosity was studied at the test temperatures of 40 • C and 100 • C. A kinematic viscosity value was assumed for each sample for comparison with the additive-free oil sample. Figure 2A,B shows the kinematic viscosity of all nano lubricant samples at the temperatures of 40 • C and 100 • C. It can be seen that the kinematic viscosity increased significantly with the increase in the hybrid filler content. Moreover, the highest value of kinematic viscosity was measured when using a higher filler content. The viscosity increases of the samples loaded with 2.0 wt.% Al 2 O 3 NPs and 0.5 wt.% MWCNTs were 6.8% and 2.8% at the temperatures of 40 • C and 100 • C, respectively. The dispersions loaded with 2.0 wt.% Al 2 O 3 NPs and 1.0 wt.% MWCNTs increased the kinematic viscosity by 8.0% and 3.5% at the test temperatures of 40 • C and 100 • C, respectively. This could have been due to the presence of nanoparticles between the oil layers, which contributed to the improvement of the viscosity. Moreover, the phenomena of agglomeration and collision of particles contribute to the increase in the viscosity of oil even at high temperatures [31]. The results obtained by many researchers are in agreement [56][57][58]. wear tracks and marks of each block were evaluated using an electronic microscope (OLYMPUS BX53M; Tokyo, Japan). The morphology of the worn surfaces was examined with a scanning electron microscope (SEM; JCM-6000Plus; JEOL, Tokyo, Japan).

Influence of Al2O3/MWCNTs on Kinematic Viscosity
The effect of the dispersion of the hybrid Al2O3/MWCNTs in lubricating oil on the kinematic viscosity was studied at the test temperatures of 40 °C and 100 °C. A kinematic viscosity value was assumed for each sample for comparison with the additive-free oil sample. Figure 2A,B shows the kinematic viscosity of all nano lubricant samples at the temperatures of 40 °C and 100 °C. It can be seen that the kinematic viscosity increased significantly with the increase in the hybrid filler content. Moreover, the highest value of kinematic viscosity was measured when using a higher filler content. The viscosity increases of the samples loaded with 2.0 wt.% Al2O3 NPs and 0.5 wt.% MWCNTs were 6.8% and 2.8% at the temperatures of 40 °C and 100 °C, respectively. The dispersions loaded with 2.0 wt.% Al2O3 NPs and 1.0 wt.% MWCNTs increased the kinematic viscosity by 8.0% and 3.5% at the test temperatures of 40 °C and 100 °C, respectively. This could have been due to the presence of nanoparticles between the oil layers, which contributed to the improvement of the viscosity. Moreover, the phenomena of agglomeration and collision of particles contribute to the increase in the viscosity of oil even at high temperatures [31]. The results obtained by many researchers are in agreement [56][57][58].  The viscosity index was used to analyze the lubricant characteristics and was determined starting from the kinematic viscosity at the test temperatures of 40 • C and 100 • C according to ASTM D2270. Figure 2C shows the viscosity index of the nano lubricant samples. It can be observed that the viscosity index of the nano lubricant increased as the loading level increased. The best improvement rates were obtained in samples with higher loading contents, with the viscosity values increasing by 32.17% and 39.7% in

Influence of Al 2 O 3 /MWCNTs on the Coefficient of Friction
The tribological performance of the nano lubricant samples was analyzed and compared with the additive-free lubricant sample. The friction coefficients of the hybrid nanofillers with different charges of Al 2 O 3 NPs and charges of 0.5% and 1.0 wt% MWCNTs are shown in Figure 3A,B. The first set of samples with Al 2 O 3 NPs and 0.5 wt% MWCNTs was investigated at different rotational speeds and temperatures. From Figure 3A, the addition of the hybrid nanofiller showed a good response to the reduction in the COF under different working conditions. Moreover, it was clear that the increase in the loading of hybrid Al 2 O 3 -NPs and 0.5 wt.% MWCNTs led to the minimization of the COF as a function of the rotational speeds. Thus, the best loading was 1.5 wt% Al 2 O 3 NPs and 0.5 wt.% MWCNTs, which reduced the COF by 45% on average, while increasing the loading above this limit did not give an improvement. Oleic acid plays an important role in the chemical reaction between filler and engine oil, which contributes to increasing the homogeneity of the mixture [59]. The mechanism of Al 2 O 3 NPs contributes to the reduction in the COF and metal-metal contact boundaries, which can be attributed to the rolling effect of nanoparticles [1,60]. Moreover, the formation of a thin film of the nanofiller leads to preventing contact with the asperities, and the self-lubricating property of MWCNTs contributes to increasing the effectiveness and decreasing the COF [61][62][63][64]. The samples were investigated at the different temperatures of 40, 70, and 100 • C, with 200 N as normal load and at 1000 rpm speed. The COF of the sample with free filler increased significantly with the increase in the temperature. This could be attributed to the fact that the increase in the temperature directly led to a decrease in the viscosity of the oil. In addition, the COF decreased with the increase in the temperature in the presence of nanofillers, with the COF decreasing by up to 58% at 100 • C. This decrease may have been due to the presence of the nanofiller directly on the contact surface, which plays the role of the rolling element and contributes to the formation of a self-lubricating film [12,65]. The same observation holds for the samples loaded with different proportions of Al 2 O 3 -NPs and 1.0 wt% MWCNTs (see Figure 4). The results of this set of samples showed that the hybrid sample loaded with 1.0 wt% Al 2 O 3 NPs and 1.0 wt.% MWCNTs had a significantly reduced COF of 47.9% compared with the base lubricant sample. It can be concluded that the COF of hybrid nano lubricants decreased due to the presence of the nanofiller, and the best results were obtained with the loading of 0.0 wt.% Al 2 O 3 NPs and 1.0 wt.% MWCNTs.

Influence of Al2O3/MWCNTs on the Wear Rate
The wear rate was evaluated in the samples loaded with hybrid Al2O3 NPs and MWCNTs as a function of the working conditions. Figure 5 shows the effect of adding the Al2O3/MWCNT hybrid nanofiller to engine oil on the wear rate at a constant load of 200 N, different sliding paths, and temperatures of 40, 70, and 100 °C. In general, a decrease in the wear rate was observed with the increase in the sliding distance, indicating increased wear resistance. Moreover, the presence of hybrid nanofillers contributes to the reduction in the wear rate at different sliding distances [1], [5]. It can be observed that the dispersed engine oil with 1.5 wt.% Al2O3 NPs and 1.0 wt.% MWCNTs, Lube-13, had the lowest wear rate. The reduction in the wear rate was up to 58% compared with the additive-free sample, Lube-00. On the other hand, an increase in the temperature led to a decrease in viscosity, which reduced wear resistance. This means that care should be taken when using this sample, Lube-00, at high temperatures. Nevertheless, the hybrid nanofiller responded well to the high temperature, which helped to increase the wear resistance. Hybrid fillers play an effective role in reducing the wear rate, which could be due to their properties, such as surface enhancement and the formation of oil nanofilm

Influence of Al 2 O 3 /MWCNTs on the Wear Rate
The wear rate was evaluated in the samples loaded with hybrid Al 2 O 3 NPs and MWCNTs as a function of the working conditions. Figure 5 shows the effect of adding the Al 2 O 3 /MWCNT hybrid nanofiller to engine oil on the wear rate at a constant load of 200 N, different sliding paths, and temperatures of 40, 70, and 100 • C. In general, a decrease in the wear rate was observed with the increase in the sliding distance, indicating increased wear resistance. Moreover, the presence of hybrid nanofillers contributes to the reduction in the wear rate at different sliding distances [1,5]. It can be observed that the dispersed engine oil with 1.5 wt.% Al 2 O 3 NPs and 1.0 wt.% MWCNTs, Lube-13, had the lowest wear rate. The reduction in the wear rate was up to 58% compared with the additive-free sample, Lube-00. On the other hand, an increase in the temperature led to a decrease in viscosity, which reduced wear resistance. This means that care should be taken when using this sample, Lube-00, at high temperatures. Nevertheless, the hybrid nanofiller responded well to the high temperature, which helped to increase the wear resistance. Hybrid fillers play an effective role in reducing the wear rate, which could be due to their properties, such as surface enhancement and the formation of oil nanofilm [51]. The effect of increasing the MWCNT loading from 0.5% to 1.0 wt.% is shown in Figure 6. It could be assumed that the wear rate decreased with the addition of 1.0 wt.% MWCNTs. This could be attributed to the increase in the MWCNT loading, which may favor the presence of particles on the fibers [66]. This may have helped to improve the dispersion of the nanofiller in the oil. From the results, dispersion of the samples with 1.0 wt% MWCNTs reduced the wear rate similar to the previous samples. However, these samples showed a significant response at a high temperature when it was set higher than the previous samples. Thus, the wear rate decreased by 70% on average at 100 • C. It was found that the addition of hybrid Al 2 O 3 /MWCNTs increased the wear resistance, and the results were also in agreement with previous results [49,53,58,62].
favor the presence of particles on the fibers [66]. This may have helped to improve the dispersion of the nanofiller in the oil. From the results, dispersion of the samples with 1.0 wt% MWCNTs reduced the wear rate similar to the previous samples. However, these samples showed a significant response at a high temperature when it was set higher than the previous samples. Thus, the wear rate decreased by 70% on average at 100 °C. It was found that the addition of hybrid Al2O3/MWCNTs increased the wear resistance, and the results were also in agreement with previous results [49,53,58,62].

Influence of Al2O3/MWCNTs on Worn Surfaces
The scar width on worn surfaces was examined to assess the lubricant mechanism of the tested samples. Figure 7 presents the wear scar width as a function of the loading content of the hybrid nanofiller under a constant applied load of 200 N at 40 °C. The results indicated that the wear scar width reduced due to the presence of the nanofiller. Compared with the additive-free sample, the wear scar width was minimized by about favor the presence of particles on the fibers [66]. This may have helped to improve the dispersion of the nanofiller in the oil. From the results, dispersion of the samples with 1.0 wt% MWCNTs reduced the wear rate similar to the previous samples. However, these samples showed a significant response at a high temperature when it was set higher than the previous samples. Thus, the wear rate decreased by 70% on average at 100 °C. It was found that the addition of hybrid Al2O3/MWCNTs increased the wear resistance, and the results were also in agreement with previous results [49,53,58,62].

Influence of Al2O3/MWCNTs on Worn Surfaces
The scar width on worn surfaces was examined to assess the lubricant mechanism of the tested samples. Figure 7 presents the wear scar width as a function of the loading content of the hybrid nanofiller under a constant applied load of 200 N at 40 °C. The results indicated that the wear scar width reduced due to the presence of the nanofiller. Compared with the additive-free sample, the wear scar width was minimized by about

Influence of Al 2 O 3 /MWCNTs on Worn Surfaces
The scar width on worn surfaces was examined to assess the lubricant mechanism of the tested samples. Figure 7 presents the wear scar width as a function of the loading content of the hybrid nanofiller under a constant applied load of 200 N at 40 • C. The results indicated that the wear scar width reduced due to the presence of the nanofiller. Compared with the additive-free sample, the wear scar width was minimized by about 36% in sample Lube-13, while the increase in filler loading did not give a good indication of the wear scar, as displayed in sample Lube-14. Moreover, it was evident that adding 1.0 wt.% MWCNTs helped to improve the wear mechanism of the lubricant. It could be observed that the maximum wear scar width was achieved for a sample dispersion of 1.0 wt.% Al 2 O 3 NPs and 1.0 wt.% MWCNTs (Lube-22), with a reduction ratio of 51.5%. 36% in sample Lube-13, while the increase in filler loading did not give a good indication of the wear scar, as displayed in sample Lube-14. Moreover, it was evident that adding 1.0 wt.% MWCNTs helped to improve the wear mechanism of the lubricant. It could be observed that the maximum wear scar width was achieved for a sample dispersion of 1.0 wt.% Al2O3 NPs and 1.0 wt.% MWCNTs (Lube-22), with a reduction ratio of 51.5%.

Influence of Al2O3/MWCNTs on Topography
For better illustration, the topography of the worn contact surfaces of the nano lubricant samples is shown in Figure 8 for the loading levels of 0.5% and 1.0% by weight MWCNTs, respectively. The topography of the Lube-00 sample showed the presence of many wear marks and the propagation of the grooves and wear residues. In addition, the 3D topography showed the depth of these traces and the roughness of the friction surface. Moreover, the dispersion of the hybrid nanofiller led to a reduction in the wear marks and their depth, which could have been due to the formation of tribofilm performed by the nano lubricant. It could be observed that the sliding surfaces of the samples filled with Al2O3 NPs and 0.5 wt.% MWCNTs became less rough. Moreover, it could be observed that an oil film was deposited on the sliding surface. This could have been the main reason for the decreases in the COF and the wear rate. The same observations could be confirmed by examining samples with Al2O3 NPs and 0.5 wt% MWCNTs, as shown in Figure 9. Finally, the topography of the sliding surfaces showed that the samples with the nanofiller had cracks, grooves, wear marks, and deposits, and the surface roughness was lower than that of the sample without additive. Figure 10 shows the SEM images of the nano lubricant samples. To obtain a clear analysis, all samples were scanned at the same magnification factor. The SEM image of the pure engine oil sample showed clear damage to the sample surface in the form of delamination and scuffing. In contrast, the surfaces of the nano lubricant samples were less damaged and smoother. The surfaces with the least damage were found in Lubricant 13 and Lubricant 22 samples. It could be concluded that these samples had smooth surfaces without wear marks. It could be assumed that the MWCNTs contributed to the improvement of the wear mechanism because they formed tribofilm that separated and prevented metal-metal contact [66], while the Al2O3 NPs played an effective role as nano-rolling elements that reduced the COF and repaired the worn surfaces [1], [53].

Influence of Al 2 O 3 /MWCNTs on Topography
For better illustration, the topography of the worn contact surfaces of the nano lubricant samples is shown in Figure 8 for the loading levels of 0.5% and 1.0% by weight MWCNTs, respectively. The topography of the Lube-00 sample showed the presence of many wear marks and the propagation of the grooves and wear residues. In addition, the 3D topography showed the depth of these traces and the roughness of the friction surface. Moreover, the dispersion of the hybrid nanofiller led to a reduction in the wear marks and their depth, which could have been due to the formation of tribofilm performed by the nano lubricant. It could be observed that the sliding surfaces of the samples filled with Al 2 O 3 NPs and 0.5 wt.% MWCNTs became less rough. Moreover, it could be observed that an oil film was deposited on the sliding surface. This could have been the main reason for the decreases in the COF and the wear rate. The same observations could be confirmed by examining samples with Al 2 O 3 NPs and 0.5 wt% MWCNTs, as shown in Figure 9. Finally, the topography of the sliding surfaces showed that the samples with the nanofiller had cracks, grooves, wear marks, and deposits, and the surface roughness was lower than that of the sample without additive. Figure 10 shows the SEM images of the nano lubricant samples. To obtain a clear analysis, all samples were scanned at the same magnification factor. The SEM image of the pure engine oil sample showed clear damage to the sample surface in the form of delamination and scuffing. In contrast, the surfaces of the nano lubricant samples were less damaged and smoother. The surfaces with the least damage were found in Lubricant 13 and Lubricant 22 samples. It could be concluded that these samples had smooth surfaces without wear marks. It could be assumed that the MWCNTs contributed to the improvement of the wear mechanism because they formed tribofilm that separated and prevented metal-metal contact [66], while the Al 2 O 3 NPs played an effective role as nano-rolling elements that reduced the COF and repaired the worn surfaces [1,53].   Compared to our work on Al2O3/MWCNTs, several nanoparticles have been reported in the literature. For instance, ZnO NPs were dispersed in diesel oil to evaluate the thermal and tribological properties. The results showed that an oil sample supplemented with 0.7 wt% ZnO NPs contributed to a decrease in the purity point and weight a loading amount of 1.0 wt% TiO2 NPs and 0.5 wt% SiO2 NPs. Moreover, the wear rate and the COF decreased by 47% to 39% compared with additive-free oil. The COF decreased by 86% with engine oil filled with 0.3 wt% TiO2 NPs under mixed and boundary lubrication conditions [40]. Engine oil 10W-30 with suspended TiO2 NPs was also investigated using a four-ball tribometer at 75 °C. The TiO2 NPs were filled with different amounts of 0.01%, 0.025%, 0.050%, and 0.075% by weight [41]. In the wear scar results, the sample filled with 0.075 wt% TiO2 NPs showed the best tribological performance.  Compared to our work on Al 2 O 3 /MWCNTs, several nanoparticles have been reported in the literature. For instance, ZnO NPs were dispersed in diesel oil to evaluate the thermal and tribological properties. The results showed that an oil sample supplemented with 0.7 wt% ZnO NPs contributed to a decrease in the purity point and weight loss of 15.2% and 86%, respectively, compared with the pure oil sample [30]. In addition, MoS 2 and ZnO NPs were used as nanofillers for diesel oil. The samples were tested at an operating temperature of 100 • C to evaluate the loading degree. The results showed that the oil viscosity increased by 9.58% and 10.14% with the content of 0.7 wt% MoS 2 and ZnO NPs, respectively. It was possible to confirm that ZnO NPs is a more suitable option for dispersion in diesel oil [31]. Moreover, polyol ester oil filled with ZnO NPs was investigated to study the tribological and thermal properties at different operating temperatures [32]. It was found that at a content of 0.3 wt% of ZnO NPs, the pour point decreased by 13.6%; the flash point increased by 3.5%; and the COF decreased by 7%. Moreover, base oil dispersed with MoS 2, and SiO 2 NPs was used as a lubricant between magnesium alloy and steel contacts. It was observed that the best results were obtained with 0.1% and 0.7% by weight MoS 2 and SiO 2 NPs, respectively [33]. In addition, engine oil dispersed with TiO 2 and SiO 2 NPs (0.5 and 1.0 wt.%) was investigated. The experiments were evaluated using a tribometer under different operating conditions, such as load, speed, and temperature [38,39]. The best results were obtained for samples with a loading amount of 1.0 wt% TiO 2 NPs and 0.5 wt% SiO 2 NPs. Moreover, the wear rate and the COF decreased by 47% to 39% compared with additive-free oil. The COF decreased by 86% with engine oil filled with 0.3 wt% TiO 2 NPs under mixed and boundary lubrication conditions [40]. Engine oil 10W-30 with suspended TiO 2 NPs was also investigated using a four-ball tribometer at 75 • C. The TiO 2 NPs were filled with different amounts of 0.01%, 0.025%, 0.050%, and 0.075% by weight [41]. In the wear scar results, the sample filled with 0.075 wt% TiO 2 NPs showed the best tribological performance.

Conclusions
The high thermal conductivity and mechanical properties of Al 2 O 3 nanoparticles and MWCNTs help to dissipate heat generated by friction, which helps the lubricant to maintain its viscosity, ultimately resulting in less metal-to-metal contact and reduced friction and wear. In the present work, the tribological properties and viscosity of lubricating oil 10W-30 with Al 2 O 3 /MWCNT hybrid nanofiller were investigated. From the experimental results, it could be concluded that (1)  (4) the nano lubricant samples responded well to the increase in the temperature, resulting in a 58% decrease in the COF at 100 • C compared with base oil; (5) the wear rate was also reduced by 58% at 40 • C, while the reduction was up to 70% at 100 • C. Moreover, the width of wear scar decreased by 36% and 51.5% in samples loaded with 1.5 wt.% Al 2 O 3 and 0.5 wt.% MWCNTs and 1.0 wt.% Al 2 O 3 and 1.0 wt.% MWCNTs, respectively. Based on these results, it is recommended that Al 2 O 3 /MWCNT hybrid nanofiller for engine oil 10W-30 be used effectively at high operating temperatures. Moreover, the loading amount of 2.0 wt% Al 2 O 3 /MWCNTs in a ratio of 1:1 is the optimal dispersion amount.